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Morishita, Yuki; Yamada, Tsutomu*; Nakasone, Takamasa*; Kanno, Marina*; Sasaki, Miyuki; Sanada, Yukihisa; Torii, Tatsuo*
Radiation Measurements, 188, p.107502_1 - 107502_7, 2025/11
Times Cited Count：0The decommissioning of the Fukushima Daiichi Nuclear Power Station requires thorough inspection of piping for contamination, including alpha nuclides. Since external alpha particle measurements are impractical, detection relies on gamma-rays emitted by the alpha nuclides. Therefore, a phoswich detector for detecting low-energy gamma-rays was developed and experimentally validated. The detector was designed with consideration of energy deposition characteristics and consists of YAP:Ce or HR-GAGG scintillators in combination with BGO scintillators, employing a photomultiplier tube for signal amplification. Validation procedures included Monte Carlo simulations and measurements using actual radiation sources. Both measurement and simulation results demonstrate a correlation in scintillator energy depositions across different gamma-ray energies. Pulse Shape Discrimination (PSD) plots effectively differentiate between low-energy and high-energy gamma-rays, thereby confirming the predictions from simulations. These results suggest promising potential for developing a sensitive low-energy gamma-ray detector utilizing various scintillator combinations. The phoswich detector shows promise for effectively detecting low-energy gamma-rays emitted by alpha nuclides in piping.
Morishita, Yuki; Peschet, L.; Yamada, Tsutomu*; Nakasone, Takamasa*; Kanno, Marina*; Sasaki, Miyuki; Sanada, Yukihisa; Torii, Tatsuo*
Radiation Measurements, 183, p.107414_1 - 107414_6, 2025/04
Times Cited Count：1 Percentile：0.00(Nuclear Science & Technology)In the decommissioning nuclear facilities, it is crucial to inspect piping for contamination to prevent worker exposure to alpha-emitting nuclides. Traditional methods using gamma rays and neutrons are inadequate for detecting small amounts of alpha nuclides due to the short range (approximately 4 cm) of alpha particles in air. To address this, we developed a compact detector capable of distinguishing between alpha particles for direct measurement within pipes. This detector, comprising a ZnS(Ag) scintillator for alpha particles and a plastic scintillator for beta particles (gamma rays), was coupled to a small photomultiplier tube. The system demonstrated high accuracy in differentiating between alpha and beta radiation through pulse shape discrimination (PSD). Monte Carlo simulations and empirical measurements confirmed the detector's effectiveness, achieving a 51.3% detection efficiency for alpha particles with negligible sensitivity to beta and gamma radiation. This innovation presents a significant advancement for direct alpha contamination measurement in environments with high beta and gamma backgrounds, such as the Fukushima Daiichi Nuclear Power Plant decommissioning site.
Sato, Rina; Yoshimura, Kazuya; Sanada, Yukihisa; Mikami, Satoshi; Yamada, Tsutomu*; Nakasone, Takamasa*; Kanaizuka, Seiichi*; Sato, Tetsuro*; Mori, Tsubasa*; Takagi, Marie*
Environment International, 194, p.109148_1 - 109148_8, 2024/12
Times Cited Count：2 Percentile：23.68(Environmental Sciences)Assessment of individual external doses from ambient dose equivalents is used for predictive and retrospective assessments where personal dosimeters are not applicable. However, it tends to contain more errors than assessment by personal dosimetry due to various parameters. Therefore, in order to accurately assess the individual dose from ambient dose equivalents, a model that estimates effective doses considering life patterns and the shielding effects by buildings and vehicles, were developed in this study. The model parameters were examined using robust datasets of environmental radiation measured in the areas affected by the Fukushima Daiichi Nuclear Power Station accident in 2020 to 2021. The accuracy of the model was validated by comparison with 106 daily personal doses measured in Fukushima Prefecture in 2020. The measured personal dose was well reproduced by the model-estimated effective dose, showing that the model can be used to assess the individual exposure dose, similar to personal dosimetry. Furthermore, this model is an effective tool for radiation protection, as it can estimate the individual dose predictively and retrospectively by using environmental radiation data.
Sanada, Yukihisa; Abe, Tomohisa; Sasaki, Miyuki; Kanno, Marina*; Yamada, Tsutomu*; Nakasone, Takamasa*; Miyazaki, Nobuyuki*; Oshikiri, Keisuke*; Watabe, Hiroshi*
Journal of Nuclear Science and Technology, 61(5), p.693 - 702, 2024/05
Times Cited Count：7 Percentile：86.26(Nuclear Science & Technology)The "treated water" from which the main radioactive materials were removed contains tritium, and stakeholders in Japan have been debating how to treat it. The amount of treated water stored in the facility has reached its limit, and the Japanese government has decided that the treatment method will be to discharge the water into the ocean by FY 2023 (FY: Fiscal Year). The present research developed a simple and practical tritium monitor for the Fukushima Daiichi Nuclear Power Station (FDNPS) tritiated water release. A simple and practical tritium monitor was developed for FDNPS tritiated water release based on thin plastic scintillator sheets. The developed devices were calibrated using standard tritium solutions and a method for calculating the minimum detectable activity. Fifteen 0.25 mm-thick scintillators can be placed in the 0.26 L flow cell where the sample water is fed, yielding an active surface area of 3,200 mm
. The efficiency of tritiated water with full water is 0.000035 cps Bq
. The minimum detectable activity under simple shielding conditions was 7,800 Bq L (Measurement time was 3,600 s).
Sanada, Yukihisa; Miyamoto, Kenji*; Momma, Hiroyasu*; Miyazaki, Nobuyuki*; Nakasone, Takamasa*; Tahara, Junichiro*; Baba, Shoichiro*; Furuyama, Hiroki*
Marine Technology Society Journal, 55(5), p.222 - 230, 2021/09
Large quantities of volatile radionuclides were released into the atmosphere following the Fukushima Daiichi Nuclear Power Station (FDNPS) accident in March 2011. To evaluate their effect on aquatic organisms and radiocesium behavior in the environment, the monitoring of radioactive cesium in the waterbed soil is important. We developed a dedicated radiation survey device that can both directly measure and collect some sediment samples. A comprehensive test was conducted using this device, which was installed in an unmanned surface vehicle at Fukushima offshore contaminated by radiocesium that resulted from the FDNPS accident. Consequently, the effectiveness of this device was confirmed because the measurement results of the sediment-core sample and in-situ radiation survey results show a good correlation. A successful application of this device in the time of post-nuclear facility accident is expected.
(Ce) detector used for estimating radiocesium distribution in soilMalins, A.; Ochi, Kotaro; Nakasone, Takamasa*; Yamada, Tsutomu*; Machida, Masahiko; Kurikami, Hiroshi; Saito, Kimiaki; Sanada, Yukihisa
no journal, ,
Here we report PHITS Monte Carlo simulations of a LaBr(Ce) gamma spectroscopy system used in unmanned helicopter radiation surveys in Fukushima Prefecture. The simulated spectra were verified against measurements taken under controlled laboratory conditions. Simulations were used to characterize the peak-to-Compton method for estimating the vertical distribution of radiocesium in soil from airborne surveys.
Morishita, Yuki; Kanno, Marina*; Abe, Tomohisa; Sasaki, Miyuki; Terasaka, Yuta; Yamada, Tsutomu*; Nakasone, Takamasa*
no journal, ,
This study presents the development of a low-cost, compact continuous air monitor (CAM) capable of measuring both alpha and beta radiation in high gamma-ray dose environments. Traditional CAMs are expensive and become unusable after contamination, posing challenges for decommissioning sites. This new CAM utilizes an inexpensive plastic scintillator detector coupled with a photomultiplier tube, allowing for energy spectrum measurement and particle discrimination. The detector can be easily replaced when contaminated, significantly reducing operational costs. The developed CAM was integrated with an existing cordless dust sampler (CDS), enabling portable and continuous monitoring. Performance testing demonstrated the ability to accurately measure both alpha and beta radiation, as evidenced by energy spectra analysis. Continuous monitoring in a high radon concentration environment successfully captured time-series variations of alpha particles from radon progeny. This low-cost and portable CAM offers a valuable tool for contamination monitoring in various contaminated sites.
(Ce) detector for estimating radiocesium distribution in soilOchi, Kotaro; Malins, A.; Ishida, Mutsushi*; Yamada, Tsutomu*; Nakasone, Takamasa*; Sanada, Yukihisa
no journal, ,
no abstracts in English
Sasaki, Miyuki; Yamada, Tsutomu*; Nakasone, Takamasa*; Sanada, Yukihisa
no journal, ,
no abstracts in English
Morishita, Yuki; Yamada, Tsutomu*; Nakasone, Takamasa*; Kanno, Marina*; Sasaki, Miyuki; Sanada, Yukihisa; Torii, Tatsuo*
no journal, ,
We have tried to create a novel compact alpha/beta detector capable of direct insertion into piping for the immediate assessment of alpha and beta contamination. The detector, a phoswich detector, comprises scintillators for alpha and beta particles. For alpha particles, a ZnS(Ag) scintillator with a thickness of 3.25 mg/cm was used as the initial layer, while a cerium-doped GdGaAl
O
(GGAG) scintillator measuring 6 mm 
6 mm 6 mm served as the scintillator for the secondary layer for beta particles. The bottom of the GAGG scintillator was coupled with optical grease to a compact photomultiplier tube (H14603, Hamamatsu Photonics K.K.), with an effective diameter of 8 mm
. The signal was amplified by a preamplifier embedded in the compact photomultiplier tube and then the signal was input to a digitizer (DT5730S, CAEN). to confirm the alpha/beta discrimination performance. Subsequently, a thorium source emitting alpha and beta particles concurrently conducted measurement. A charge integration (CI) method was employed to distinguish the alpha and beta waveforms. A 2-dimensional Pulse Shape Discrimination (PSD) map showed the distribution of two clusters of alpha and beta particles, both of which were effectively discriminated. The detector has outer dimensions of 22.0 mm 22.0 mm 30.0 mm, facilitating direct insertion into a pipe for measurement purposes. As a future endeavor, we aim to optimize the discrimination between alpha and beta particles by modifying the combination of scintillators.
Morishita, Yuki; Yamada, Tsutomu*; Nakasone, Takamasa*; Kanno, Marina*; Sasaki, Miyuki; Sanada, Yukihisa; Torii, Tatsuo*
no journal, ,
The Fukushima Daiichi Nuclear Power Station's decommissioning requires thorough inspection of piping for contamination, including alpha nuclides. External alpha particle measurements are impossible, necessitating gamma-ray detection methods. A phoswich detector for low-energy gamma-ray detection was developed and validated experimentally at a nuclear fuel facility. The detector was designed considering energy deposition characteristics. The detector comprises YAlO:Ce and Bi
GeO scintillators with a photomultiplier tube for signal amplification. Measurement and simulation results show correlation between scintillator energy depositions for different gamma-ray energies. Pulse Shape Discrimination (PSD) plots distinguish alpha particles and gamma-rays, confirming simulation predictions. Nuclear fuel facility measurements exhibit higher energy deposition in the first scintillator layer. This suggests potential for developing a sensitive low-energy gamma-ray detector using scintillator combinations. Future work aims to optimize detector design and conduct field tests for validation.
